Die Entdeckung des Higgs-Bosons im Jahr 2012 bestätigt das Standardmodell als die erfolgreichste Theorie, die die grundlegenden Wechselwirkungen von Elementarteilchen beschreibt. Eine der wichtigen Eigenschaften des Higgs-Bosons ist seine Yukawa-Kopplung an das Top-Quark, die aufgrund der hohen Masse des Quarks im Standardmodell am stärksten ist. Diese Arbeit berichtet über eine Messung der Top-Yukawa-Kopplung mit Daten, die vom ATLAS-Detektor von 2015 bis 2018 bei einem Massenschwerpunkt von 13 TeV aufgezeichnet wurden. Die Kopplung wird in ttH(bb)-Ereignissen untersucht, einem Endzustand, der die Zerfälls-produkte von zwei Top-Quarks enthält und in dem zusätzlich ein Higgs-Boson emittiert wird, welches in Bottom-Quark-Paar zerfällt. Dieser Zerfallskanal des Higgs-Bosons hat das größte Verzweigungsverhältnis, wird jedoch durch die Beschreibung des dominanten Untergrundprozesses ttbb, ein Top-Quark-Paar mit zwei zusätzlichen b-Quarks im Endzustand systematisch beschränkt. Die Messung nutzt die Fähigkeit des ATLAS-Detektors, Jets von einem b-Quark zu identifizieren, um Analysebereiche mit verschiedenen Zusammensetzungen von Signal und Untergrund zu konstruieren. Um das Signal weiter zu separieren, wird eine Reihe von multivariaten Algorithmen verwendet und der ttH-Prozess wird unter Verwendung eines Profile-Likelihood-Fits extrahiert. Die Ergebnisse werden für den Kanal mit einem einzelnen Lepton im Endzustand und für eine Kombination mit dem Dilepton-Kanal gezeigt. Die Untergrundgenauigkeit wird im Detail untersucht, wobei große Fehlmodellierungen festgestellt werden. Das gemessene Verhältnis der ttH-Produktion zur Standardmodell-Vorhersage beträgt mu(ttH) = 0,84+0,45- 0,39 (syst.) +-0,21 (stat.). Das Ergebnis stimmt mit der Vorhersage des Standardmodells überein und entspricht einer beobachteten (erwarteten) Signifikanz von 1,9 sigma (2,3 sigma), eine Verbesserung gegenüber der vorherigen ATLAS-Messung, bei der eine Signifikanz von 1,4sigma (1,6 sigma) ermittelt wurden. / The discovery of the Higgs Boson in 2012 confirms the Standard Model as the most successful theory describing the fundamental interactions of elemental particles. One of the important properties of the Higgs boson is its Yukawa coupling to the top quark, which in the Standard Model is the strongest due to the high mass of the quark. This thesis reports on a measurement of the top-Yukawa coupling with data collected by the ATLAS detector from 2015 to 2018 at 13 TeV center of mass energy. The coupling is studied in ttH(bb) events, a final state containing decay products of two top quarks with additional emission of a Higgs boson, where the Higgs decays into a pair of bottom quarks. This decay channel of the Higgs Boson has the largest branching ratio, but is systematically limited by the description of the dominant background process ttbb, a tt with additional two b quarks in the final state. The measurement takes advantage of the ability of the ATLAS detector to identify jets coming from a b quarks to construct analysis regions with various compositions of the signal and the background. To further separate the signal, a series of multivariate algorithms is employed and the ttH process is then extracted using a profile likelihood fit. The results are shown for the channel with a single lepton in the final state and for a combination with the dilepton channel. The background performance is studied in detail, where large mis-modeling is found. The measured ratio of the ttH production compared to the Standard Model prediction is found to be mu(ttH) = 0.84 +0.45 -0.39 (syst.) +-0.21 (stat.). The result is in agreement with the Standard Model prediction and corresponds to an observed (expected) significance of 1.9 sigma (2.3 sigma), an improvement compared to the previous ATLAS measurement which reported 1.4 sigma (1.6 sigma).

With the start of the Large Hadron Collider(LHC) at CERN, we will obtain a new understanding of the physics beyond our current limits. New discoveries will be made; but we will require a deeper understanding, which the LHC machine, being a hadron collider, will not be able to elucidate. Instead, we will need an e+e- collider to make precision measurements of the newly discovered phenomena. Electroweak symmetry breaking and the origin of fermion and boson masses are fundamental issues in our understanding of particle physics. The essential piece of electroweak symmetry breaking - the Higgs boson - will probably be discovered at the LHC. If there are one, or more, Higgs boson(s) precise measurements of all properties of the Higgs will be very important. In this thesis I present two measurements of Standard Model Higgs boson properties in the context of the International Linear Collider (ILC) at √s = 500 GeV, using the proposed International Linear Detector (ILD). First a performance study of ILD to measure the branching ratios of the Higgs boson with mH = 120 GeV, where the Higgs boson is produced with a Z-boson via the Higgsstralung process, and the Z decays into e+e- or μ+μ-. It will also be essential to study the Higgs Yukawa coupling. Therefore, in the second part of this thesis, I present a study of e+e- → tt¯H with the aim of making a direct measurement of the the top-Higgs coupling, using the semi-leptonic nal state and mH of 120 GeV. I show that the top-Higgs coupling can be measured with an accuracy of better than 28%.

The Higgs boson and the top quark have been a focus in modern elementary particle physics research because of their special roles in the Standard Model (SM) of particle physics. The studies of both particles are crucial for revealing the unsolved puzzles of modern particle physics. The coupling between the Higgs boson and the top quark, the top-Yukawa coupling, is one of the fundamental parameters in the SM that can potentially direct the future development of the theory of elementary particle physics. This thesis presents two analyses on the Higgs boson and the top quark, using proton-proton (pp) collision data collected by the ATLAS detector during 2012 and 2015. A search for the SM Higgs boson produced in association with a top quark pair (tt̄H) was performed using 20.3 fb⁻¹ of pp collision data at a centre-of-mass energy of √s = 8 TeV. The search is designed to be primarily sensitive to the H → bb decay mode. Events with one of two electrons or muons are used in the search. No significant excess of events is observed above the background predicted by the SM. An observed (expected) upper limit on the signal strength of 3.4 (2.2) times the SM prediction is obtained at 95% confidence level. The tt̄H signal strength, represented by the ratio of the measured tt̄H cross-section to the SM prediction, is found to be μ = 1.5 ± 1.1 for a Higgs boson mass of m_H = 125 GeV. A measurement of the top quark pair (tt̄) production cross-section was performed using 3.2 fb⁻¹ of pp collision data at √s = 13 TeV. The measurement uses events with an opposite-charge-sign electron-muon pair and exactly one and two jets originating from b quarks. The inclusive tt̄ production cross-section is measured to be σ_tt̄ = 818 ± 8(stat) ± 27(syst) ± 19(lumi) ± 12(beam) pb, where the uncertainties arise from data statistics, analysis systematic effects, the integrated luminosity and the LHC beam energy. The total relative uncertainty is 4.4%. The result is consistent with the theoretical prediction at the next-to-next-to-leading order accuracy in the strong coupling constant αs of QCD, with the resummation of next-to-next-to-leading logarithmic (NNLL) soft gluon terms. A fiducial cross-section corresponding to the experimental acceptance of leptons is also measured.

Submitted by Luciana Ferreira (lucgeral@gmail.com) on 2016-02-22T12:27:05Z No. of bitstreams: 2 Dissertação - Luiz Eduardo de Sousa Freire - 2015.pdf: 1113525 bytes, checksum: 2604d25df22a0fce378e1f9840262c72 (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Approved for entry into archive by Luciana Ferreira (lucgeral@gmail.com) on 2016-02-22T12:28:32Z (GMT) No. of bitstreams: 2 Dissertação - Luiz Eduardo de Sousa Freire - 2015.pdf: 1113525 bytes, checksum: 2604d25df22a0fce378e1f9840262c72 (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Made available in DSpace on 2016-02-22T12:28:32Z (GMT). No. of bitstreams: 2 Dissertação - Luiz Eduardo de Sousa Freire - 2015.pdf: 1113525 bytes, checksum: 2604d25df22a0fce378e1f9840262c72 (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Previous issue date: 2015-03-20 / Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq / In this paper we investigate two-dimensional membranes, such as graphene, using Quantum Field Theory, more specifically by the path integral formalism and using Hubbard- Stratonovich transformations. With graphene as a motivation, we use models for electrons in a graphene layer, when they interact with phonons, Kekule deformation and gauge fields. We start by considering a theory in which bosons and fermions interact via a Yukawa type coupling. We present a method already known to eliminate the degrees of freedom of the fermionic system and use the Hubbard-Stratonovich transformations to derive an effective bosonic theory. For this, we introduce an auxiliary field in the model and show that the effective theory for this field is equivalent to the effective theory for the physical bosonic field. Thus, we calculated and obtained the gap equations for this system in 1+1, 2+1 and 3+1 dimensions and then compared with the Gross-Neveu model for quartic interactions between fermions in 1+1 dimensions. We see that for a particular coupling constant the massive electrons lose all their mass by interacting with bosons, an effect caused by symmetry breaking. We then present the chiral gauge model of Jackiw-Pi for graphene, where Yukawa type interactions are present. However, this theory is a particular case of a more general model proposed by Frederico et al. where bosonic self-interactions at higher orders and bosons/fermions with more general interactions are considered. Again, we use the Hubbard- Stratonovich transformations to derive effective models for fermions and the gap equations. We identified chiral invariance transformations of the group U(1) for the limiting case of Jackiw-Pi model. Finally, we investigated a model for phonons in the graphene background, more specifically building on the papers by Katsnelson et al. Guinea et al.. At this point, anharmonic terms are included in the Lagrangian of the system in an attempt to describe changes in the background structure. We eliminate the degrees of freedom of the scalar system, which are responsible for describing power modes on and off plane and thus obtained an effective theory for the electronic part. As in the previous model, we see a change in the effective potential and derive their gap equations. Finally, we present the Coulomb potential to derive an effective theory for fermions when they interact with a gauge field. Thus, we compare this result with the studied models and analyzed the effective fermionic interactions present in the obtained Lagrangians. / Neste trabalho investigamos membranas bidimensionais, tal como o grafeno, usando Teoria Quântica de Campos, mais especificamente pelo formalismo de integrais de trajetória e usando transformações de Hubbard-Stratonovich. Com o grafeno como motivação, usamos modelos para os elétrons em uma camada de grafeno, quando estes interagem com fônons da rede, deformações de Kekulé e campos de gauge. Começamos analisando uma teoria em que bósons e férmions interagem entre si por meio de um acoplamento tipo Yukawa. Apresentamos um método já conhecido para eliminar os graus de liberdade fermiônicos do sistema e usamos as transformações de Hubbard-Stratonovich para derivar uma teoria efetiva bosônica. Para isto, inserimos um campo auxiliar no modelo e mostramos que a teoria efetiva pra este campo é equivalente à teoria efetiva para o campo bosônico físico. Assim, calculamos e simulamos as equações de gap para este sistema em 1+1, 2+1 e 3+1 dimensões e comparamos com o modelo de Gross-Neveu para interações quárticas entre férmions em 1+1 dimensões. Vemos que para dada constante de acoplamento, férmions massivos perdem toda sua massa ao interagir com os bósons, efeito causado por quebra de simetria. Em seguida, apresentamos o modelo de gauge quiral de Jackiw-Pi para o grafeno, onde interações tipo Yukawa estão presentes. Contudo, uma generalização é proposta por Frederico et al. em que auto-interações bosônicas de ordens mais altas e interações bósons/férmions mais gerais são consideradas. Novamente, usamos as transformações de Hubbard-Stratonovich para derivar modelos efetivos para os férmions e as equações de gap. Identificamos a invariância por transformações quirais do grupo U(1) para o caso limite do modelo de Jackiw-Pi. Em seguida, investigamos um modelo para os fônons da rede no grafeno, mais especificamente, a partir dos trabalhos de Katsnelson et al e Guinea et al. Neste ponto, termos anarmônicos são incluídos na lagrangiana do sistema no intento de descrever modificações na estrutura da rede. Eliminamos os graus de liberdade escalares do sistema, que são aqueles responsáveis por descrever os modos de energia dentro e fora do plano e, assim, obtivemos uma teoria efetiva para a parte eletrônica. Como no modelo anterior, inferimos uma mudança no potencial efetivo e derivamos as respectivas equações de gap. Por fim, apresentamos o potencial de Coulomb ao derivar uma teoria efetiva para férmions quando estes interagem com um campo de gauge. Assim, comparamos este resultado com os modelos estudados e analisamos as interações fermiônicas efetivas presentes nas lagrangianas obtidas.

Utilisant des données Monte-Carlo ATLAS en simulation complète du canal ttH;H->WW(*), cette thèse prsésente une étude de la mesure du couplage de Yukawa du quark top pour une lumisosité intégrée de 30fb(-1) dans la gamme de masses du Higgs allant de 120 à 200GeV. Les effets du système de déclenchement, de l'empilement des événements, ainsi que de toutes les erreures systématiques possibles ont été étudiés. Pour une masses du Higgs de 160GeV, en incluant les erreures systématiques, la significance obtenue du signal est à 2 sigma en combinant les états finaux à deux et trois leptons. Le rapport d'embranchement combiné sigma_ttH*Br(H->WW(*)) peut atteindre une précision de 47 %.Cette thèse comprend également une étude de la linéarité de la réponse à des électrons VLE avec des données de tests en faisceau combiné d'un secteur du détecteur ATLAS. Les résultats MC utilisant une graine multiple 5X5 permettent d'envisager une amélioration future de la linéarité de réponse à des électron VLE.

Le boson de Higgs est l'une des particules elementaires predites par le modele standard, et la seule pas encore observee experimentalement. La recherche de la particule de Higgs est l'objectif premier de nombreuses experiences à haute energie au cours des dix dernieresannees. Selon le modele standard, les bosons W, Z0 bosons obtiennent leurs masses de la brisure spontanee de la symetrie du vide par le mecanisme de Higgs, tandis que les fermions obtiennent leurs masses par couplage de Yukawa avec le boson Higgs. Ce couplage est une des plus importantes proprietes du Higgs et permet de distinguer si il est de type modele standard ou non. Le Grand Collisioneur de Hadron (Large Hadron Collider ou LHC) au CERN est un collisionneur proton-proton avec une energie dans le centre de masse de 14 TeV. ATLAS est un detecteur de particules de type generaliste situe aupres d'un des points de collision du LHC. Le debut de la prise des donnees est prevue pour cet ete. Les principaux objectifs de physique de l'experience ATLAS sont les recherches du boson de Higgs, de particules SUSY et les mesures de precision electro-faibles. Utilisant des donnees Monte-Carlo ATLAS CSC (Computing System Commissioning) en simulation complµete du canal t¹tH;H ! WW, cette these presente une etude de la mesure du couplage de Yukawa du quark top pour une lumisosite integree de 30fb 1 dans la gamme de masses du boson de Higgs allant de 120 a 200GeV=c2. Pour la premiµere fois, les effets du systeme de declenchement, de l'empilement des evenements, ainsi que de toutes les erreurs systematiques possibles ont ete extensivement etudies. Pour une masses du Higgs de 160GeV=c2, en incluant toutes les erreures systematiques, la signicance obtenue du signal est superieure µa 2¾ en combinant les etats naux a deux et trois leptons. Le rapport d'embranchement combine ¾ t¹tH BrHWW peut atteindre une precision de 47 % et donner des informations importantes sur le couplage de Yukawa du quark top. C'est la premiere etude du canal t¹tH;H WW en simulation detaillee du detecteur ATLAS, incluant une etude complete et detaillee des erreurs systematiques. La partie la plus dicile de l'analyse de t¹tH;H WW est d'extraire le signal du bruit de fond abondant car la section efficace du signal est seulement 0:1% de celle du bruit de fond tt. En outre, le signal presente un etat final complexe d'au moins quatre jets, deux leptons et deux neutrinos qui rendent tres diffcile la reconstruction du spectre de masse du Higgs. L'isolation leptonique est un des moyens les plus importants pour supprimer le bruit de fond reductible. Cette these developpe une procedure dediee d'isolation en cone permettant de supprimer le bruit principal t¹t par un facteur 5. L'incertitude sur l'echelle d'energie leptonique est l'une des principales sources d'incertitude systematique pour cette analyse t¹tH;H WW. Une bonne linearite de reponse pour des electrons de tres basses energies (VLE) permettrait d'ameliorer la performance de l'estimation de l'echelle d'energie des electrons. Cette these comprend egalement une etude de la linearite de la reponse à des electrons VLE avec des donnees de tests en faisceau combine (CTB) d'un secteur du detecteur ATLAS. Une premiere methode basee sur l'utilisation, evenement par evenement, de chambreà fils proche du faisceau est presentee. Puis une autre methode est etudiee par simulation Monte-Carlo. Elle est basee sur l'utilisation d'un etalonnage s'appuyant sur une constante de calibration obtenue par une aggregation des energies par graine multiple de 5 par 5. Les resultats de simulation de cette methode permettent d'envisager une amelioration future de la linearite de reponse à des electrons VLE.

CARVALHO, João Cláudio Nunes. Estruturas, modos normais e difusão em um sistema periódico quasi-unidimensional. 2011. 111 f. Tese (Doutorado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2011. / Submitted by francisco lima (admir@ufc.br) on 2014-03-17T13:26:12Z No. of bitstreams: 1 2012_tese_jcncarvalho.pdf: 3563614 bytes, checksum: a2a41532f12b4d1dae87a73ae73cb8c4 (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2014-03-17T22:16:32Z (GMT) No. of bitstreams: 1 2012_tese_jcncarvalho.pdf: 3563614 bytes, checksum: a2a41532f12b4d1dae87a73ae73cb8c4 (MD5) / Made available in DSpace on 2014-03-17T22:16:32Z (GMT). No. of bitstreams: 1 2012_tese_jcncarvalho.pdf: 3563614 bytes, checksum: a2a41532f12b4d1dae87a73ae73cb8c4 (MD5) Previous issue date: 2012 / In this thesis we analyze the structural properties and diffusive dynamics for a classical system of two-dimensional (2D) charged particles, interacting through a repulsive Yukawa potential of the type exp(−r/ lambda)/r, and confined in a parabolic channel that limits the movement of particles in the y direction. Along the x direction, the particles are subject to a periodic potential. The ground-state configurations and the normal mode spectra are obtained in terms of the periodicity (L), the intensity of the periodic potential (V0) and density. An interesting set of tunable ground-state configurations are found, with first- or second-order structural transitions between them. A configuration with particles aligned, perpendicular to the x direction, in each minimum of the periodic potential is obtained for V0 larger than some critical value that has a power-law dependence on the density. The phonon spectrum of different configurations was also calculated. A localization of the modes into a small frequency interval is observed for sufficiently large strength of the periodic potential, and a tunable gap in the phonon spectrum is found as a function of V0. Langevin dynamic simulations are used to investigate the effect of the particle density, the amplitude of the periodic substrate, and the range of the interparticle interaction potential on the diffusive behavior of the particles. We found that in general the diffusion is suppressed with increasing amplitude of the periodic potential, but for specific values of the strength of the substrate potential a remarkably increase of the diffusion is found with increasing periodic potential amplitude. In addition, we found a strong dependence of the diffusion on the specific arrangement of the particles, e.g. singlechain versus multi-chain configuration. For certain particle configurations, a reentrant behavior of the diffusion is found as function of the substrate strength due to structural transitions in the ordering of the particles. / Nesta tese analisamos as propriedades estruturais, dinâmicas e difusivas para um um sistema clássico bidimensional (2D) onde partículas carregadas interagem através de um potencial do tipo Yukawa repulsivo exp(−r/λ)/r, e estão confinadas em um canal do tipo parabólico, o qual limita o movimento das partículas na direção y. Ao longo da direção x, as partículas estão sujeitas a um potencial periódico. As configurações do estado fundamental e o espectro dos modos normais são obtidos em função: da periodicidade (L), da intensidade do potencial periódico (V0) e da densidade. Um interessante conjunto de estados fundamentais são encontrados, com transições estruturais de primeira ou de segunda entre eles. Uma configuração com partículas alinhadas, perpendicular à direção x, em cada mínimo do potencial periódico é obtida para valores de V0 maiores do que um determinado valor crítico, observa-se uma dependência de V0 em relação à densidade na forma de uma lei de potência. O espectro de fônons para diferentes configurações também foi calculado. A localização dos modos em um pequeno intervalo de frequência é observada para uma intensidade do potencial periódico suficientemente grande do, e um intervalo(gap) ajustável no espectro fônon é encontrado em função de V0. A difusão de partículas carregadas interagindo por meio de simulações de Langevin foram utilizadas para investigar o efeito da da densidade de partículas, da amplitude do substrato periódico, e o intervalo do potencial de interação entre as partículas sobre o comportamento difusivo das partículas. Descobrimos que, em geral, a difusão é suprimida com o crescimento da amplitude do potencial periódico, mas para valores específicos da intensidade do potencial do substrato um aumento notável na difusão é encontrado com o aumento da amplitude do potencial periódico. Além disso, encontramos uma forte dependência da difusão em relação ao arranjo das partículas (uma cadeia versus duas cadeias). Para determinados arranjos das partículas, um comportamento reentrante na difusão é encontrado em função da intensidade do substrato devido as transições estruturais na ordenação das partículas.

This thesis summarizes knowledge about mycotoxins, with focus to ochratoxin A. It also summarizes its tolerable levels of food intake, detoxification and analytical methods for mycotoxins. The work also includes a chapter describing liposomes that were used for the analysis of ochratoxin A by liposomal electrokinetic capillary electrophoresis (LECK). The practical part includes the analysis of ochratoxin A from Aspergillus ochraceus Wilhelm and Aspergillus melleus Yukawa fungi cultivated on a rye and optimization of the method for analysis of ochratoxin A based on liposomes of different compositions. By capillary zone electrophoresis, ochratoxin A is not sufficiently separated and detected in the extracted mixture; conversely, when liposome solutions are used, different migration behavior can be achieved while stabilizing ochratoxin A in solution due to amphiphilic interactions between mycotoxins and liposomes. Therefore, the LEKC method was used for this work. The most suitable liposome composition has been shown to be molar ratios of 25% cholesterol (membrane stabilization) / 50% 2-oleoyl-1-palmitoyl-sn-glycerol 3-phosphocholine (main zwitterionic lipid) / (25% 1,2-diacyl-sn-glycerol)-3-phospho-L-serine (introduction of negative charge).

La découverte du boson de Higgs au LHC en 2012 est la plus récente confirmation de la validité du modèle standard, théorie décrivant les particules élémentaires et leurs interactions fondamentales. De nombreuses études ont été mises en place pour étudier les différentes caractéristiques de cette particule récemment découverte. Ce travail se concentre sur la recherche du processus ttH dans l’expérience ATLAS, pour réaliser une première mesure directe du couplage de Yukawa du boson de Higgs au quark top, paramètre important dans les modèles de physique au-delà du modèle standard. La première partie de ce travail a porté sur l’automatisation de l’étalonnage du calorimètre hadronique à tuiles du détecteur ATLAS (TileCal ) par un système laser. Une description de ce système qui permet un étalonnage régulier de toutes les cellules du calorimètre est tout d’abord présentée. Ensuite, l’algorithme d’automatisation de ce processus d’étalonnage, qui a été écrit lors de ce travail, est décrit. Le but final de cet algorithme est de faciliter et accélérer la correction des canaux dont le gain dérive. Une deuxième partie concerne la recherche du processus ttH par l’étude des états finals multi-leptons et particulièrement avec deux leptons de même charge électrique et au moins 4 jets dont 1 étiqueté b. L’estimation des bruits de fond instrumentaux et le traitement statistique réalisé sont décrits en détail dans le document pour deux versions de l’analyse. Une première version correspondant aux données de l’année 2015 et du début 2016, soit une luminosité intégrée de 13.2f b −1 de données, aboutit à une précision insuffisante pour conclure sur la présence du processus ttH. Une deuxième version de l’analyse, optimisée avec l’utilisation d’analyses multivariées sur l’ensemble des données 2015 et 2016, se conclut par une observation du processus ttH lors de la combinaison de l’ensemble des états finals ttH. La signification statistique observée est alors de 4.2σ pour une signification statistique attendue de 3.8σ. Ce résultat est donc en accord avec la prédiction du modèle standard. / Discovery of Higgs boson at LHC in 2012 is the most recent confirmation of the validity of Standard Model, theory describing elementary particles and their interactions. Many analysis now target the extraction of properties of the newly-discovered particle. A direct measurement in the ATLAS experiment of the top Yukawa coupling, one of these properties, is targeted in this work through ttH process. This coupling is of particular interest because of its strong sensitivity to New Physics. The first part of the work is about the automation of the calibration of the hadronic tile calorimeter of the ATLAS detector. A detailed description of the laser system used for a regular calibration of the calorimeter is done as well as of the calibration itself. Then more details on the algorithm written for the automation of the calibration are given. The final goal of this algorithm is to ease and fasten the calibration of channels with gain variation. The second part is dedicated to the search for ttH process through multilepton final states with emphasis on final state with two same-sign leptons, at least four jets and at least 1 b-tagged jet. Estimation of reducible backgrounds and statistical treatment of the analysis are detailed. A first version of the analysis with 13.2f b −1 , corresponding to 2015 and mid-2016 LHC data, ends with a final precision too low to extract any conclusion on the tt̄H process. In a second version of the analysis, improvements are made using multivariate analysis and adding more signal regions. The results from the multilepton analysis is combined with results from other ttH analysis, targeting other Higgs decays, to attain a final observed sentivity of 4.2σ. Thus an evidence for ttH production can be claimed from this combination. The final results give good agreement with Standard Model prediction.

L'un des principaux objectifs de l'experience ATLAS est la recherche de la derniere particule manquante du modele standard, le boson de Higgs, et l'etude de ses proprietes. Cette experience sera placee aupres du futur collisionneur de protons LHC, dont le demarrage est prevu pour l'annee 2007 au CERN. Les travaux presentes dans cette these sont dedies a la recherche d'un boson de Higgs de masse inferieure a 200 GeV/c2, dans le canal de production associee pp->ttH. L'identification des jets contenant des hadrons beaux est fondamentale pour l'etude du boson de Higgs et du quark top. Les performances d'etiquetage des jets b sont etudiees en details dans le canal ttH, H->bb, et pour des conditions de fonctionnement tres proches de celles attendues au demarrage des prises de donnees, (detecteur a pixels incomplet, bruit d'empilement et inefficacites de detection). Pour une efficacite d'identifcation de jets b de 60%, les taux de rejet attendus sont de 80% pour les jets legers et 7% pour les jets c. Si la decouverte du boson de Higgs dans ATLAS semble relativement aisee, la mesure de ses parametres, et en particulier la determination des couplages aux differents fermions, est beaucoup plus delicate. Une etude de faisabilite est proposee pour un nouveau canal, ttH, H->WW(*). Ce canal est le seul qui permet une determination directe du couplage de Yukawa du quark top dans l'intervalle de masse 130-200 GeV/c2. Pour un boson de Higgs de 160 GeV/c2, la precision statistique attendue apres trois annees de prise de donnees a haute luminosite est de 7 %.